Aluminum alloy is a metallic material emerged lately, and had not been applied industrially until the beginning of the 20th Century. During the period of World War II, aluminum materials was mainly used to produce military aircrafts. After the war, as the demand for aluminum materials in the military industry decreased suddenly, the community of aluminum industry set about to develop aluminum alloy for civil use; therefore, the fields of application of aluminum alloy expanded from aircraft industry to all sectors of national economy such as building industry, vessel packaging industry, traffic and transport industry, electric power and electronic industry, mechanical manufacturing industry, and petrochemical industry, etc., and the aluminum alloy was gradually applied in people's daily life. Nowadays, aluminum materials is only inferior to iron and steel materials in terms of application scale and scope, and become the second major metallic material in the world.
From the aspect of manufacturing and aluminum alloy products, high-strength aluminum alloys are usually divided into wrought aluminum alloys and cast aluminum alloys; from the aspect of working temperature of the products, high-strength aluminum alloys are divided into ordinary aluminum alloys and high-temperature (or heat-resistant) aluminum alloys. Up to now, only Al—Cu based aluminum alloys can meet the demand for high temperature and high strength features. Viewed from designation series, Al—Cu based aluminum alloys comprises cast aluminum alloys and wrought aluminum alloys, both of which belong to Series 2 aluminum alloys; however, there is no publication to disclose the high-temperature aluminum alloy with high strength which has good casting properties and tend to deforming machining.
1. High-strength Cast Aluminum Alloy and Wrought Aluminum Alloys
In generally, cast aluminum alloys include AlSi based aluminum alloy, AlCu based aluminum alloy, AlMg based aluminum alloy, and AlZn based aluminum alloy, wherein, AlCu based aluminum alloy and AlZn based aluminum alloy have the highest strength, but most of them have a strength in the range of 200 MPa˜300 MPa.
Only a few of designations from the AlCu based aluminum alloy have a strength higher than 400 MPa, but the cost of manufacture of them is high, since it is required of refined aluminum matrix and admixture of noble elements; AlZn based cast alloys have poor heat-resistant performance. Therefore, the scope of application of ordinary cast aluminum alloys is severely limited because these alloys have inferior obdurability when compared to wrought aluminum alloys. For important application purposes, such as load wheels for special heavy duty vehicles and aviation applications, usually wrought aluminum alloys are used, instead of cast aluminum alloys. By means of extrusion, rolling, and forging, etc., wrought aluminum alloys have reduced defects, refined crystal grains, and increased tightness, and therefore have high strength, excellent toughness, and high service performance. However, owing to the high requirement for processing equipment and molds and complex processing procedures, wrought aluminum alloys require a long production cycle and high cost. Compared with wrought aluminum alloys, cast aluminum alloys have advantages such as lower price, isotropic structure, availability of special structures, applicability for production of components with complicated shapes, and suitability for small-lot production and mass production, etc. Therefore, it is of great theoretical significance and high practical application value to develop cast aluminum alloy materials with high-obdurability and cast forming processes for replacement of some wrought aluminum alloys, so as to attain the purpose of replacing forging with cast, shortening manufacturing cycle, and reducing production cost.
In the developing process of cast aluminum alloys with high-obdurability, the A-U5GT cast aluminum alloy developed in France at the beginning of the 20th Century takes an important place. Among typical cast aluminum alloys with high-obdurability available presently, A-U5GT has the longest history and the widest scope of application. There is no corresponding designation equivalent to it in China yet.
American Aluminum Association designation 201.0 (1986) and 206.0 (1967), which were developed on the basis of A-U5GT, have excellent mechanical properties and stress corrosion resistant property. However, since they contain 0.4%˜1.0% of silver, they have a high material cost and are only applied in military field or other demanding fields, with a limited scope of application.
China has achieved remarkable achievements in the field of cast aluminum alloy with high-obdurability. In 1960s to 1970s, Beijing Aviation Material Institute successfully developed ZL205A alloy. ZL205A alloy has a complex composition, containing seven kinds of alloying elements, i.e., Cu, Mn, Zr, V, Cd, Ti, and B. ZL205A (T6) has a tensile strength of 510 MPa, which is the highest among the registered designations of cast aluminum alloy materials. ZL205A (T5) has the highest obdurability and an elongation up to 13%. However, a major defect of ZL205A is its poor casting properties and high tendency of hot cracking; in addition, it has a small scope of application due to the high cost of formulation.
The above three cast aluminum alloys with high-obdurability belong to Al—Cu base having high strength as well as high plasticity and toughness. However, their casting properties are not so satisfactory, represented by high tendency of hot cracking, poor flowability, and poor feeding property. Moreover, Al—Cu based alloys have poor corrosive resistance and exhibit a tendency of intercrystalline corrosion. The finished product rate of the Al—Cu based alloys in the casting process is very low.
In the four published patent applications Nos. 200810302670.3, 200810302668.6, 200810302669.0, and 200810302671.8, titled as “High-Strength Cast Aluminum Alloy Material”, a high-strength cast aluminum alloy material composed of Cu, Mn, Ti, Cr, Cd, Zr, B, and rare earth elements was disclosed. The aluminum alloy material has a tensile strength up to 440 MPa and an elongation greater than 6%; however, in actual application of the high-strength cast aluminum alloy material, the problems of high tendency to hot cracking and severe contradiction between alloy strength and castability are not solved, mainly because of the wide temperature range of quasi-solid phase within the composition range of major elements Cu and Mn of the alloy, which provides sufficient conditions for growth of anisotropic dendritic crystals during solidification in the casting process, and therefore results in high internal shrinkage stress in the late stage of solidification and leaves high tendency to hot cracking during shrinkage.
Up to now, there are more than 70 kinds of formally registered designations of wrought aluminum alloy in Series 2XXX, and most of them are registered in USA, wherein, only 14 designations (i.e., 2001, 2004, 2011, 2011A, 2111, 2219, 2319, 2419, 2519, 2021, 2A16, 2A17, 2A20, and 2B16) are high-copper aluminum alloys with a copper content of higher than 5%, and only 4 kinds of designations (i.e., 2A16, 2A17, 2A20, and 2B16) have a copper content of higher than 6%. These wrought aluminum alloys have high contents of Si, Mg, and Zn, etc. in their formulations, but there is no micro-alloying elements such as rare earth (RE) elements. Therefore, their formulations are much different from those of the Series 2 cast aluminum alloys, which reflects the difference in production process and deep processing process between the two types of aluminum alloys.
2. High-Temperature Aluminum Alloys
High-temperature alloys are also referred to heat-resistant alloys with high-strength, thermal-strength alloys, or super alloys, which is an important metallic material developed as the emergence of the aviation turbine engines in the 1940s. They can withstand high service load for a long period under the condition of high temperature oxidative atmosphere and exhaust corrosion, are mainly applied for hot-side components of gas turbine, and is an important structural material in aerospace and aviation, ship, power generation, petrochemical, and transportation industries. Wherein, some alloys can also be applied as materials in arthrosteopedic surgery and dental surgery in biological engineering field.
Common high-temperature alloys include nickel-based, iron-based, and cobalt-based alloys, which can service in high-temperature environments at 600˜1100° C.; whereas, heat-resistant aluminum alloys were developed in the cold war period. Heat-resistant aluminum alloys with high-strength are suitable to bear high service load in hot environments up to a temperature of 400° C. for a long period, and are more and more applied in aerospace and aviation, and heavy-duty mechanical industries, etc. Strong-power components subjected to high-temperature and high-pressure can be cast from heat-resistant aluminum alloys with high-strength, except for the components that directly contact with high temperature fuel gas in aviation turbine engines and gas turbines, etc.
Owing to the fact that aluminum alloys are easy to process, as the improving of the technical level of processing, wrought aluminum alloys are used to replace cast aluminum alloys in more and more applications, provided that the requirement for strength is met. Therefore, heat-resistant aluminum alloys with high-strength are divided into cast alloys and wrought alloys.
Usually, heat-resistant alloys with high-strength contain several or even tens of alloying elements. The admixed elements perform the functions such as solid solution strengthening, dispersion strengthening, grain boundary strengthening, and surface stabilization in the alloy, to enable the alloy to maintain high mechanical properties and high environmental performance at high temperature.
Considerations in Selection of High-Temperature Alloy for Casting:
(1) Normal, maximum, and minimum working temperatures and temperature fluctuation rate of the cast product;
(2) Temperature difference range of the cast product and expansion property of the alloy;
(3) Characteristics of the load on the cast product, and loading, supporting, and external constraints;
(4) Requirement for service life of cast product, allowable amount of deformation, nature of working environment, shaping method, and factors related to cost, etc.
At present, in the Chinese national standards, aluminum alloy materials for casting of high temperature parts only include designations of A201.0, ZL206, ZL207, ZL208, and 206.0, including Al—Cu—Mn based alloys and Al-RE based alloys; wherein, most of Al—Cu—Mn based alloys employ high-purity aluminum ingots as the alloy material, and therefore have a high cost; whereas the Al-RE based alloys have a relatively poor mechanical properties at room temperature. Moreover, most heat-resistant aluminum alloys with high-strength available today have drawbacks such as low strength at high temperature (instantaneous tensile strength less than 200 MPa and long-term strength less than 100 MPa at a temperature of 250° C. or higher), high formulation cost, poor casting properties, low casting yield rate, and poor reuse of waste scrap and slag, etc., resulting in poor quality of cast products, high cost, and long slag treatment cycle, etc. Furthermore, most heat-resistant aluminum alloys declared for patent application in recent years contain noble elements in their formulations, and have unsatisfactory casting properties, can not meet the technological progress in aviation industry in terms of quality, and are unsuitable for industrial production and application.
Few heat-resistant wrought aluminum alloys with high-strength that can be widely applied in the development of national economy and modernization of national defense and have a splendid prospect are seen in domestic or foreign literature. Most of known Series 2XXX wrought aluminum alloys (such as 2219, 2A02, 2A04, 2A06, 2A10, 2A11, 2A12, 2A14, 2A16, 2A17, 2A50, 2A70, and 2A80, etc.) and Series 7XXX wrought aluminum alloys (such as 7A04, etc.) have a strength lower than 100 MPa at a temperature of 256° C. or higher, and the major micro-alloying elements are Si, Mg, and Zn, besides Cu and Mn. There is no report on the heat-resistant wrought aluminum alloy materials with high-strength having a strength of higher than 150 MPa at a temperature of 250° C. or higher without admixture of those elements.
In summary, the problems existing in the research of heat-resistant aluminum alloys with high-strength in China and foreign countries include: insufficient strength and durability at high temperature, instantaneous strength less than 250 MPa at a temperature of 250° C. or higher, and long-term strength less than 100 MPa at high temperature; poor processability of the material, long waste treatment cycle, high cost, and lag behind the technological progress in aviation industry, etc.